Carbon nanotubes are a novel form of carbon. Single-wall carbon nanotubes are hollow, tubular fullerene molecules consisting essentially of sp2-hybridized carbon atoms typically arranged in hexagons and pentagons. Single-wall carbon nanotubes typically have diameters in the range between about 0.5 nanometers (nm) and about 3.5 nm, and lengths usually greater than about 50 nm. They are known for their excellent electrical and thermal conductivity and high tensile strength. Since their discovery in 1993, there has been substantial research to describe their properties and develop applications using them.
Multiple-wall carbon nanotubes, also called multi-wall carbon nanotubes, are nested single-wall carbon cylinders. The number of walls in a multi-wall carbon nanotube can be as few as two (double-wall carbon nanotube) or three (triple-wall carbon nanotube) and may range up to hundreds. Multi-wall carbon nanotubes possess some properties similar to single-wall carbon nanotubes. However, as the number of walls increases, so does the number of defects. Because single-wall carbon nanotubes generally cannot accommodate defects during growth, they typically have very few defects. The minimal number of defects usually renders single-wall carbon nanotubes stronger and more conductive than multi-wall carbon nanotubes. Single wall carbon nanotubes are known to readily form into “ropes”, which are aggregates of multiple parallel tubes in contact with one another. The single-wall carbon nanotubes in the ropes are cohesively held tightly together by strong van der Waals forces. Besides ropes of single-wall carbon nanotubes, ropes of small-diameter carbon nanotubes (i.e. diameters between 0.5 nm and 3 nm) have been observed with nanotubes having single and multiple walls. Such carbon nanotube ropes of small-diameter carbon nanotubes are illustrated in “Catalytic Growth of Single-Wall Carbon Nanotubes from Metal Particles,” International Pat. Publ. WO 00/17102 A1, published Mar. 30, 2000. Large multi-wall carbon nanotubes, with diameters greater than about 4 nm, tend to have an increasing number of defects and decreasing electrical conductivity and tensile strength. The larger, less-flexible multi-wall carbon nanotubes also do not form “ropes”.
Most methods for carbon nanotube production involve one or a combination of transition metal catalysts in contact with a carbon-containing feedstock at an elevated temperature typically between about 700° C. and 1200° C. Some of the methods to make carbon nanotubes include electric arc, laser ablation of graphite, and gas phase techniques with supported and unsupported metal catalyst.
One method of preparing carbon nanotubes on supported metal catalyst is known as “chemical vapor deposition” or “CVD”. In this method, gaseous carbon-containing feedstock molecules react on nanometer-scale particles of catalytic metal supported on a substrate to form carbon nanotubes. This procedure has been used to produce multi-wall carbon nanotubes, however, under certain reaction conditions, it can produce excellent single-wall carbon nanotubes. Synthesis of small-diameter carbon nanotubes using CVD methodology has been described in Dai, et al. (1996), Chem. Phys. Lett., 260, p. 471-475, and “Catalytic Growth of Single-Wall Carbon Nanotubes from Metal Particles,” International Pat. Publ. WO 00/17102 A1, published Mar. 30, 2000, each incorporated herein by reference. The carbon nanotube material that results from a CVD process comprises single-wall and small-diameter multi-wall carbon nanotubes, residual catalyst metal particles, catalyst support material, and other extraneous carbon forms, which can be amorphous carbon, and non-tubular fullerenes. The term “extraneous carbon” will be used herein as any carbon that is not in the form of carbon nanotubes, and can include graphene sheets, non-tubular fullerenes, partial nanotube forms, amorphous carbon and other disordered carbon.
Many end-use applications for carbon nanotubes require that the nanotube material be effectively dispersed in another medium such as a liquid solvent solution or molten material in order to form a composite comprising nanotubes and a matrix material which can be polymeric, metallic, organic, inorganic or combinations thereof. When carbon nanotubes are dispersed in a matrix material, the physical, electrical, chemical and thermal properties of the composite material can be different compared to those of the matrix material alone. The properties of the nanotube composites depend, in part, on the concentration of nanotubes in the composite and on the diameter, length and morphology of carbon nanotubes in the matrix material. For example, when blending carbon nanotubes in liquids, the length distribution of the nanotubes can affect the viscosity characteristics of the liquid/nanotube mixture. The properties of the composite are highly dependent on how effectively the carbon nanotubes are dispersed in the composite. There is a substantial need for carbon nanotube materials that can easily be dispersed in matrix materials, and particularly those that can be dispersed by commercially-effective methods such as melt blending. Additionally, there is a need for a carbon nanotube material that is stable in oxidative environments at high temperatures, such as up to about 550° C.